It is a moment that many of the world's leading physicists have long yearned for. Finally, this summer, the world's largest and most expensive scientific machine will come to life hundreds of metres below the rolling countryside of the Swiss-French border.

Known as the Large Hadron Collider (LHC), it is a colossal particle accelerator measuring more than 27km round and costing more than £3bn. And by smashing together protons at almost the speed of light, it is expected to give insights into events not seen since moments after the birth of the cosmos. In the process, it will provide the ultimate test of the so-called standard model, the grand theory created over the past half-century by physicists to account for the particles of matter and the forces at work on them.

Despite its prosaic name, the standard model is one of the greatest intellectual achievements of humanity. It has revealed the underlying unity of the forces that bind together atoms and nuclei, and of the particles that make up matter. Its predictions about the nature of the subatomic world have been verified with astonishing precision, and a host of Nobel prizes awarded to its creators. Yet there remains one last prediction, one with the power to plunge theoretical physics into crisis if it fails to be confirmed by the LHC.

At its core is one of the most basic questions in all physics: what is mass? It is a question with special significance for theorists searching for their holy grail: a single, unified account of all the forces in the cosmos. Any such theory must explain the huge difference in the range of such forces, from the nuclear forces, whose influence barely extends beyond the atomic nucleus, to the electromagnetic force, which spreads to the very edge of the cosmos. Theorists have shown that the range of any force is intimately linked to the mass of the so-called exchange particles carrying the force from place to place. Just as a cannonball is harder to throw than a tennis ball, the shorter the range of a force, the more massive the exchange particle must be.

The very short range of the nuclear force thus implies it is carried by relatively hefty exchange particles. In contrast, the vast range of electromagnetism means its exchange particles - known as photons - have zero mass.

Sea of energy

From the mid-1950s onwards, theorists made repeated attempts to unify the so-called weak nuclear force with the electromagnetic force. Yet they were stymied by the problem of explaining why the masses of the exchange particles involved were so different. By the mid-1960s, an answer had begun to emerge in the form of an invisible "sea" of energy that fills the entire cosmos, known as the Higgs field, after Peter Higgs of Edinburgh University, one of the theorists who proposed the idea. Put simply, while all particles are immersed in this Higgs field, not all are influenced by it. Those that are - such as the carriers of the weak nuclear force - become imbued with the property we call mass, while those that aren't affected, such as the photon, remain mass-free.

The Higgs field was seized on by theorists as it allowed them to unify electromagnetism with the weak nuclear force and make predictions about how the resulting "electroweak" force would manifest itself in particle accelerators. These predictions have now been vindicated, and have helped make the standard model one of the triumphs of modern physics.

There is, however, a problem - and a very big one. Just as electromagnetic fields are carried by photons, the Higgs field must have its own exchange particle, known as the Higgs boson. Yet to this day, this final but crucial component of the standard model has never been found.

The Higgs boson now tops the most-wanted list for the international team operating the Large Hadron Collider. To find it, they have built colossal detectors into the LHC as tall as six-storey office blocks and weighing thousands of tonnes. These will sift through the subatomic debris left over from the billions of collisions taking place inside the machine every second, looking for the telltale signs of the Higgs boson. The proverbial needle in a haystack does not begin to capture the size of the task: according to theorists, just one Higgs boson is likely to show up in every 10,000 billion collisions.

That, of course, is presuming the particle exists at all. While many theorists hope the LHC will supply this final piece to the imposing edifice of the standard model, some fear they may be disappointed. Failure to find the Higgs boson should not come as a complete surprise, however, as it has long been clear that the standard model cannot be the ultimate description of nature.

Most obviously, it fails to encompass the most familiar of all the fundamental forces of nature: gravitation. Theorists have also long been concerned that its equations contain around 20 "free parameters" - factors whose values have defied all explanation, and which simply have to be assumed in order to get the right answers.

If the LHC fails to find the Higgs boson, no amount of sticking-plaster will be able to hold together the standard model. It will have become just one more stepping stone en route to an ultimate theory of everything.